1. Field of the Invention
The present invention relates to an apparatus, such as color image scanner, for recording an image and a method thereof, and more particularly, it relates to a method of and an apparatus for recording a halftone image while electronically generating halftone dot patterns.
2. Description of the Prior Art
FIG. 1 is a block diagram showing the structure of a halftone image scanner of the prior art. The image scanner comprises a reading drum 2 and a recording drum 5. A color original 1 is wound around the reading drum 2, and photosensitive material 4, such as a photosensitive film, is wound around the recording drum 5. The reading drum 2 and the recording drum 5 are rotated synchronously by motors 3 and 6 along directions .theta. and .phi., respectively.
While the drums 2 and 5 are rotating, a pick-up head 7 is driven by a motor 9 and a feed screw 11 at a prescribed speed along a subscanning direction X parallel to the axial direction of the reading drum 2. The pick-up head 7 reads the original 1 with respect to successive scanning lines along a circumferential direction (or a main scanning direction Y) of the reading drum 5. The pick-up head 7 includes a photoelectric unit for generating color separation signals R, G and B for red, green and blue, respectively, in response to the color tone and density of each pixel on the original 1. The color separation signals R, G and B are supplied to a signal processing unit 100 to be converted therein into a light modulation signal S.sub.d, as will be described later in detail.
While the pick-up head 7 is reading the original 1, a recording head 8 is driven by a motor 10 and a feed screw 12 at a prescribed speed along a subscanning direction U parallel to the axial direction of the recording drum 5. The recording head 8 exposes the photosensitive material 4 and records color separation images according to the light modulation signal S.sub.d.
The motor 6 is connected to an encoder 13 which generates a clock pulse K in proportion to the angle of rotation of the recording drum 5. The number of clock pulses K indicates the position of the recording head 8 along a main scanning direction V. The motor 10 is connected to an encoder 14 which generates a clock pulse L in proportion to the axial movement of the recording head 8. The number of clock pulses L indicates the position of the recording head 8 along a subscanning direction U.
The color separation signals R, G and B are converted into the light modulation signal S.sub.d in the signal processing unit 100 as follows. The color separation signals R, G and B are supplied to a color computation circuit 15. The color computation circuit 15 executes color correction and gradation correction on the color separation signals R, G and B to convert them into image signals S.sub.i which correspond to yellow magenta cyan and black colors. The clock pulses K and L from the rotary encoders 13 and 14 are supplied to a coordinate value generator 16 where they are converted into coordinate values U and V. The coordinate values U and V are defined along the subscanning direction U and the main scanning direction V, respectively, and expressed in a unit of pixel on the recording drum 5. The coordinate values U and V are supplied from the coordinate value generator 16 to an address generator 17 and converted into reading address signals S.sub.a1 and S.sub.a2. The reading address signals S.sub.a1 and S.sub.a2 are supplied from the address generator 17 to a pattern data memory 18. The pattern data memory 18 stores halftone pattern data S.sub.h representing threshold data which are compared with the signals S.sub.i to produce halftone signals S.sub.d for yellow magenta cyan and black printers. The halftone pattern data S.sub.h is read out from the pattern data memory 18 in response to the reading address signal S.sub.a1 and S.sub.a2, and supplied to a comparator 19. The comparator compares the halftone pattern data S.sub.h and the values of the image signals S.sub.i supplied from the color computation circuit 15, and generates the halftone dot signal S.sub.d for each pixel in accordance with the result of the comparison. The halftone dot signal S.sub.d is finally supplied from the comparator 19 to the recording head 8 as a light modulation signal.
The recording head 8 turns a light beam on and off in response to the light modulation signal S.sub.d to thereby expose the photosensitive material 4. In this manner, color separation images are recorded on the photosensitive material 4 with respect to each scanning line.
When the halftone image scanner described above is utilized to produce color separation images for the halftone gravure process, the following problem occurs. In carrying out the process in an inverted halftone gravure, different screen angles are applied to a solid area and to a halftone area in a single printing plate. Usually, color printing utilizes four printing plates for yellow, magenta, cyan and black printers. Halftone areas in the four printing plates are produced with different screen angles, e.g. 90.degree., 15.degree., 75.degree. and 45.degree. apart. On the other hand, solid areas in the four printing plates are all produced with a single screen angle of 45.degree.. The screen angle of the solid areas is chosen to prevent a doctor blade, which removes excessive ink on a printing plate, from becoming oriented parallel to the walls which separate the hollowed cells in the printing plate. If the doctor blade becomes parallel to the walls, excessive ink is not removed properly, causing the printed sheet to be mottled. In producing one of the four printing plates, a first halftone film is produced for the solid area while the screen is manually set at about 45.degree.. A second halftone film is produced for the halftone area with the halftone image scanner while the screen angle is automatically set by the scanner. Finally, the first and second halftone films are manually combined to form an overall halftone film with which the printing plate is produced. The above-described steps for producing the overall halftone film are laborious and inefficient.